The invention relates to the incorporation of silica powder and polybenzimidazole (PBI) polymer fibers into an elastomeric insulation for rocket motor casings.
The combustion of a solid rocket propellant charge creates a hostile environment characterized by extremely high temperature, pressure, and turbulence in the interior of the rocket motor. Temperatures typically exceed 5,000 degrees Fahrenheit (2760 degrees Celsius). Interior pressure may typically exceed 1,500 psi (1.02.times.10.sup.5 g/cm.sup.2). Chamber gas velocity typically may reach Mach 0.2 at the aft end of the rocket motor. This environment is considered particularly hostile because the combustion gas typically contains dispersed particles consisting essentially of aluminum oxide liquid droplets. These suspended droplets are believed to produce erosion of the rocket motor case insulating by a sandblasting effect. While the combustion of rocket propellant is usually of short duration, the conditions described above can destory unprotected rocket motor casings.
Those parts of the rocket structure which are exposed to the high temperatures, pressures, and erosive flow conditions generated by the burning propellant are usually protected by applying a lining of insulation. Various materials, both filled and unfilled, have been tried as insulation. These include phenolic resins, epoxy resins, high temperature melamine-formaldehyde coatings, ceramics, polyester resins and the like. These materials, when cured, usually become rigid structures which crack or blister when exposed to the rapid temperature and pressure changes occurring when the propellant is burned. Some rubber-like elastomeric systems have been tried as insulation. These systems are ablative insulation because the elastomer is sacrificed or consumed during combustion, but nevertheless provides some protection for the rocket chamber.
The best rocket insulation materials previously known to the art are elastomeric polymers reinforced with asbestos. Such systems are capable of enduring for a time sufficient to allow complete combustion of the propellant. Asbestos-reinforced elastomeric insulation systems are the subject of U.S. Pat. No. 3,421,970, to Daley et al., issued Jan. 14, 1969, and U.S. Pat. No. 3,347,047, to Hartz et al., issued Oct. 17, 1967. Those patents are hereby incorporated herein by reference.
Environmental and health concerns have led manufacturers to seek an acceptable replacement for the asbestos in rocket motor case insulation. One alternative elastomeric insulation contains aramid polymer fibers in combination with a powder filler. That insulation is disclosed and claimed in U.S. patent application Ser. No. 328,333, filed Dec. 7, 1981, now U.S. Pat. No. 4,492,779, and assigned to the owner of the present invention. The polyaramid fiber reinforced materials, however, have been found less erosion resistant than asbestos reinforced materials.
PBI fibers are known to be useful for applications requiring high temperature resistance, but have not been used within a rocket engine casing as a part of its insulation, or in an equivalent environment.